<width> (width) contains a measurement of an object along the axis parallel to its bottom,
e.g. perpendicular to the spine of a book or codex. [11.3.4 Dimensions]
If used to specify the depth of a non text-bearing portion of some object, for example
a monument, this element conventionally refers to the axis facing the observer, and
perpendicular to that indicated by the ‘depth’ axis.